Re: Somehow I have got this "thunk" thing wrong.

["Aleix Conchillo Flaqué" <aconchillo@gmail.com>]

Or you can always use eval.

((eval res (interaction-environment)))

Sorry... I was watching "The Most Beautiful Program Ever Written" talk
from W. Byrd and got too excited :-D

Aleix

On Fri, Mar 5, 2021 at 4:30 PM Olivier Dion via General Guile related
discussions <guile-user@gnu.org> wrote:
>
> On Fri, 05 Mar 2021, Tim Meehan <btmeehan@gmail.com> wrote:
> > I wanted to store a thunk in a hashtable so that I could look up its key
> > and then run it later. Something like this:
> >
> > #! /usr/bin/guile
> > !#
> >
> > (use-modules (ice-9 hash-table))
> >
> > (define stuff (alist->hash-table
> >   '((a . (lambda () (display "event a\n")))
> >     (b . (lambda () (display "event b\n")))
> >     (c . (lambda () (display "event c\n"))))))
>
> You've quoted the whole s-exp.  Which means lambda is never applied.
> You have to apply lambda in order to create a procedure.
>
> >
> > (define res (hash-ref stuff 'a))
> > (res)
> >
> > But when I run it:
> > Wrong type to apply: (lambda () (display "event a\n"))
>
> You're trying to apply a list and not a procedure.  This is because of
> the quote mentioned above.  In other words, you're doing this:
> ----------------------------------------------------------------------
> (apply '(lambda () (display "event a\n")))
> ----------------------------------------------------------------------
>
> Change the ' for ` and unquote the lambdas with ,.  Like this:
> ----------------------------------------------------------------------
> (define stuff (alist->hash-table
>   `((a . ,(lambda () (display "event a\n")))
>     (b . ,(lambda () (display "event b\n")))
>     (c . ,(lambda () (display "event c\n"))))))
> ----------------------------------------------------------------------
>
> Or use the list procedure instead of quoting:
> ----------------------------------------------------------------------
> #! /usr/bin/guile
> !#
>
> (use-modules (ice-9 hash-table))
>
> (define stuff (alist->hash-table
>   (list (cons 'a (lambda () (display "event a\n")))
>         (cons 'b (lambda () (display "event b\n")))
>         (cons 'c (lambda () (display "event c\n"))))))
>
> (define res (hash-ref stuff 'a))
> (res)
> ----------------------------------------------------------------------
>
> --
> Olivier Dion
> PolyMtl
>